An interesting character here is Prince Anton Heinrich (Antoni Henryk) Radziwill. He was Polish-Lithuanian aristocrat and since 1796 happily married to princess Louise of Prussia, a niece of Frederick the Great.
He advocating turning the Prussian gains from the second and third partition into a new Kingdom of Poland, ruled in personal union by the King of Prussia. He himself aspired probably to the role of Viceroy and de facto ruler. The Prussian defeat in 1806 and the loss of the Duchy of Warsaw ended this phase, but after 1815, he became Governor of the Grand Duchy of Posen, a Prussian province with a slightly exceptional status.
Hie political influence was weakened by his futile attempts to arrrange a marriage of his daughter and the Prussian prince Wilhelm (yes, the later king and emperor Wilhelm I.) in the 1820s. The Polish Uprising in 1830 and the fact that his own brother Michael was Commander-in-Chief of the Polish forces ended Prince Anton's career. He died in 1833.
The most plausiblechain of events might be:
Prussia does not declare ar on France in 1806, keeps the Polish parts, creates a dual monarchy of Prussia-Poland, stays on Napoleons good side and perhaps even gets Austrian New Galicia after Austria is defeated once more.
Of course, if it is widely seen as Napoleon's accomplice, once France overreaches it might be too late to change sides and a defeated Prussia-Poland is torn apart by the winners.